The present invention relates to an improved method and apparatus for depositing films of magnetic materials on a receiving substrate using the magnetron sputtering technique.
Sputtering is a well-known and widely accepted technique for depositing thin films of a desired material on a substrate. In the basic sputtering process, a target comprising the material to be deposited is bombarded by gas ions, typically argon ions, accelerated by an intense electric field. Such ion bombardment is carried out in a vacuum chamber and serves to eject, via momentum transfer, atomic sized particles from the target in all directions. Some of these particles traverse the vacuum chamber and deposit upon the substrate surface as a thin film. In order to prevent the target material from overheating during the sputtering process, conventional sputtering systems typically comprise some means of cooling the target to a relatively low temperature.
By its nature, the basic sputtering process is slow and inefficient compared with other deposition techniques; that is to say, the sputtered film accumulates at a relatively slow rate, say, 1-3 micrometers per hour, and the electric power required to produce a sputtered film is relatively high. Further, there is a tendency for the substrate to overheat and suffer radiation damage due to the lengthy film-growing procedure and the high energy of the sputtered particles and electrons.
In recent years, the above disadvantages associated with the sputtering process have been alleviated to a major extent by the development of the magnetron. Such a device comprises an elongated array of permanent magnets which is positioned behind the plane of the target material during the sputtering process. When the target is non-magnetic, magnetic lines of force emanating from the magnets pass through the target and extend into the region of the gas plasma produced by the electric field. These magnetic lines of force extend parallel to the target surface and, hence, perpendicular to the plasma-producing electric field. In cooperating with the electric field, the magnetic field above the target surface confines secondary electrons ejected from the target to the vicinity of the target surface and imparts a spiral motion thereto, thereby increasing the number of collisions such electrons have with the gas molecules of the plasma. The result in a densification of the gas plasma in the vicinity of the target surface which, in turn, acts to intensify the ion bombardment of the target and to ultimately increase the normal deposition rate by up to an order of magnitude.
While the magnetron has been used with great success in the sputtering of non-magnetic materials to produce non-magnetic films, the same cannot be said of its use in the formation of magnetic films. In attempting to magnetron sputter material from a magnetic target, one finds that the target acts as a shunt to the magnetic field lines emanating from the magnetron. Thus, the field lines which ordinarily penetrate the non-magnetic target and serve to densify the plasma near the target surface are, in effect, short-circuited through the magnetic target material and are thereby prevented from entering the region of the plasma.
While considerable thought, time and effort have been given by those skilled in the art toward providing a solution to the above-identified problem of magnetron sputtering of magnetic materials, only limited success has been achieved to date. One solution has been to use a very thin magnetic target, one so thin as to be incapable of shunting the entire magnetic field of the magnetron. Such an approach has the effect of forcing some of the magnetic flux outside the plane of the target surface and into the plasma region. The major problems with this approach, however, are that the target is relatively expensive to prepare and, owing to its thin dimension, is rapidly depleted before any substantial film can be accumulated on the receiving substrate. Another solution has been to modify the position and geometry of the permanent magnets of the magnetron. The idea is to produce a magnetic field at the surface of its target by using magnets which are spaced above and/or outside the plasma region of the vacuum chamber. This technique, however, produces a non-uniform deposition; moreover, it is difficult, at best, to produce a magnetic field of sufficient intensity at the target surface. To date, neither of these approaches has been capable of producing a sputter-deposited magnetic layer at a rate which compares favorably to the rate at which non-magnetic materials can be deposited.
In U.S. Pat. No. 4,094,761, issued to R. W. Wilson, there is disclosed a method for the magnetron sputtering of ferromagnetic material. It will be noted, however, that the object of the process disclosed is merely to provide a sputtered layer which contains a ferromagnetic material, e.g. nickel; it is not to provide a sputtered layer having magnetic properties and, in fact, the sputtered layer provided by Wilson's method is non-magnetic at room temperature. According to Wilson's method, an alloy of the desired ferromagnetic material is prepared, such alloy retaining certain desired non-magnetic characteristics and having a Curie temperature below the temperature maintained by the target prior to the sputtering process, i.e. room temperature. Upon producing the desired alloy, this alloy is used as the target material in the conventional magnetron sputtering process. Because the ferromagnetic alloy is, by design, non-magnetic at room temperature, it does not present the aforedescribed shunting problems associated with magnetic target materials.